docs: finalize documentation to the production state

The project is live in production, so the staged-development scaffolding is removed.

- Delete the staged trackers PLAN.md and PRERELEASE.md.
- Rewrite CLAUDE.md: drop the per-stage workflow; codify the ongoing development
  principles (How we work) and the production model (Branching, CI & production):
  manual prod-deploy / prod-rollback, semver release tags, Ansible provisioning,
  expand-contract migrations.
- De-stage the living docs (README, ARCHITECTURE, TESTING, deploy/ansible, loadtest,
  platform/telegram READMEs) and the docker-compose tuning comments: drop the
  Stage N / R1-R7 / pre-release labels, keep every number and rationale, and fix the
  now-dangling PLAN.md / PRERELEASE.md references to describe the current state.
- Reword stale 'later stage' Go doc comments for subsystems that have shipped.

deploy/docker-compose.prod.yml

@@ -3,7 +3,7 @@
 #
 # It (1) publishes caddy 80/443 — there is no host caddy in prod, so the contour caddy
 # owns the edge and does its own ACME on CADDY_SITE_ADDRESS — and the gateway bot-link
-# :9443 the remote bot dials in over mTLS; and (2) retunes the R7 limits down for the
+# :9443 the remote bot dials in over mTLS; and (2) retunes the baseline limits down for the
 # 2 vCPU / 1.9 GiB host (GOMAXPROCS=2, smaller memory caps, shorter Prometheus
 # retention). The contour launches deliberately undersized at zero players; the added
 # node_exporter + Grafana watch host memory so it can be resized at Selectel when
@@ -29,7 +29,7 @@ services:
     ports:
       - "9443:9443"
     environment:
-      # 2 vCPU host: align the Go scheduler with the cgroup quota (R7's 3 needs 3 cores).
+      # 2 vCPU host: align the Go scheduler with the cgroup quota (the baseline's 3-core gateway needs 3 cores).
       GOMAXPROCS: "2"
     deploy:
       resources: